theorem Th40:
  z in Ball(x,r) iff ||.x - z.|| < r
proof
  thus z in Ball(x,r) implies ||.x - z.|| < r
  proof
    assume z in Ball(x,r);
    then ex y be Point of X st z = y & ||.x - y.|| < r;
    hence thesis;
  end;
  thus thesis;
end;
